1. Field of the Invention
This invention relates to a red light-emitting phosphor capable of emitting red light at a high emission efficiency by excitation with light having a wavelength of 350 to 420 nm and also to a light-emitting device using the red light-emitting phosphor.
2. Background Art
A light-emitting diode (LED) is a semiconductor light-emitting device for light emission wherein an electric energy is converted to ultraviolet light, visible light, infrared light or the like. For instance, those making use of visible light include semiconductor light-emitters formed of light-emitting materials such as GaP, GaAsP, GaAlAs and the like, and LED lamps, in which these devices are sealed with transparent resins and the like, are now in wide use. In addition, LED lamps of a display type wherein a light-emitting material is fixed on an upper surface of a substrate or a metallic lead and is sealed with a transparent resin case shaped in the form of a numerical figure or letter has been used for various purposes.
Where a light-emitting diode that is a semiconductor device and is thus long in life and high in reliability is employed as a light source, replacement thereof is so easy that the diode has been widely used as a component part of various types of display devices such as portable communication devices, peripheral devices of personal computers, business machines, domestic electric appliances, audio devices, various types of switches, light sources of backlight, bulletin boards and the like.
Such a LED lamp may cover a wide variety of colors within a visible region of blue to red depending on the purpose in use by incorporating different types of phosphor powders in a transparent resin sealing a semiconductor light-emitter therein thereby ensuring color changes of light emitted from the LED lamp.
In recent years, however, there has been a great demand of users for color shades of such display devices. More particularly, not only a performance ensuring more precise reproducibility of minute color shades is required for display devices, but also there is a strong demand for a LED lamp which is able to emit a white color and different types of intermediate colors on its own.
To this end, attempts have been made to arrange LED lamps in such a way that different types of phosphors for red, green and blue are contained onto a surface of a semiconductor light-emitter of a LED lamp, or phosphors of such types as mentioned above are contained in a sealing material, coating material and the like of an LED lamp, thereby ensuring display of a white color and different types of intermediate colors only by a single LED lamp.
Among these phosphors, those phosphors that are excited with long wavelength UV light or short wavelength visible light (350 to 420 nm) and are currently in main use are ones including BaMg2Al16O27:Eu, or (Sr,Ca,Ba)5(PO4)3Cl:Eu whose emission color is blue, BaMg2Al16O27:Eu, Mn, or Zn2GeO4:Mn whose emission color is green, Y2O2S:Eu, La2O2S:Eu, or 3.5MgO.0.5MgF2.GeO2:Mn whose emission color is red, or the like. When these light-emitting phosphors are properly used, a wide range of light-emitting colors can be obtained.
However, the red light-emitting phosphor has a problem in that when compared with blue and green light-emitting phosphors, this phosphor is poorer in light emission against long wavelength UV light and short wavelength visible light (350 to 420 nm).
This involves problems in that where a white light-emitting color is obtained by use of light of these wavelengths, it is necessary to increase a ratio of the red light-emitting phosphor, thus inviting high costs and that white color is obtainable by balancing red, green and blue light emissions, so that in order to obtain a white light-emitting color, green and blue light emissions have to be reduced in conformity with a red light emission, and because an amount of a phosphor to be used has an upper limit, the resulting white light emission is reduced and thus, white light of a high luminance cannot be obtained. Hence, there is a demand for a red light-emitting phosphor that has a high emission efficiency and is able to emit light at high luminance.
In recent years, it has been reported that an InGaN device, which has attracted attention as a LED device capable of emitting light in long wavelength UV light and short wavelength visible light regions and ensuring high luminance light emission (Tsunemasa Taguchi, “LED display”, Journal of Illuminating Engineering Institute of Japan, Institution of Illuminating Engineering, Japan, 2003, Vol. 87, No. 1, pp. 42-47), has a light-emitting wavelength of approximately 400 nm, especially about 400 to 410 nm, at which an external quantum efficiency indicates the highest value. Thus, a red light emitting phosphor that is capable of emitting red light of high intensity within such a wavelength range as indicated above has been demanded. However, a wavelength corresponding to an excitation energy of an electron pair of an oxide compound is within a UV range. The wavelength of long wavelength UV light and short wavelength visible light (350 to 420 nm) overlaps with an absorption end of the phosphor, under which an absorption intensity at a wavelength side longer than 350 nm of these red light-emitting phosphors lowers abruptly as the wavelength becomes longer and, in fact, lowers considerably within a range not lower than 400 nm. To cope with this, it is necessary to develop a red light-emitting phosphor whose light-emitting intensity is high at a wavelength of about 400 nm.